The electric power steering apparatus (EPS) is exemplified as an apparatus that is equipped with the motor control unit. The electric power steering apparatus which provides a steering system of a vehicle with a steering assist torque (an assist torque) by a rotational torque of a motor, applies the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears by using a driving force of the motor which is controlled by electric power supplied from an inverter. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feed-back control of a motor current. The feed-back control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a handle 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a pinion-and-rack mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the steering shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the handle 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting the steering torque Th of the handle 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU: Electronic Control Unit) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist command (a steering assist command) on the basis of the steering torque Th detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor for the EPS 20 by means of a voltage control command value Vref obtained by performing compensation or the like to the current command value.
A steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle from a rotational position sensor which is connected to the motor 20.
The controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 100, and it is also possible to receive the vehicle speed Vs from the CAN. Further, a Non-CAN 41 is also possible to connect to the control unit 100, and the Non-CAN 41 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 40.
The control unit 100 mainly comprises a central processing unit (CPU) (including a micro processing unit (MPU) and a micro controller unit (MCU)), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
The control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 101 which calculates the current command value Iref1. The current command value calculating section 101 calculates the current command value Iref1, based on the steering torque Th and the vehicle speed Vs with reference to an assist map or the like, which is a control target value of a current supplied to the motor 20. The calculated current command value Iref1 is inputted into a current limiting section 103 via an adding section 102A, and the current command value Irefm whose maximum current is limited is inputted into a subtracting section 102B. A deviation ΔI (=Irefm−Im) between the current command value Irefm and a motor current value Im which is fed-back is calculated at the subtracting section 102B, and the deviation ΔI is inputted into a proportional-integral-control section (PI-control section) 104 for improving a current characteristic of the steering operation. The voltage control command value Vref that the characteristic is improved at the PI-control section 104, is inputted into a PWM-control section 105, and the motor 20 is PWM-driven through an inverter 106. The current value Im of the motor 20 is detected by a motor current detector 107 and is fed-back to the subtracting section 102B. The inverter 106 is constituted by abridge circuit of FETs as a semiconductor switching device.
The rotational sensor 21 such as the resolver is connected to the motor 20 and a motor rotational angle θ is outputted. Further, a motor velocity ω is calculated at a motor velocity calculating section 22.
A compensation signal CM from a compensation signal generating section 110 is added at the adding section 102A. A characteristic compensation of the steering system is performed by adding the compensation signal CM, and a convergence, an inertia characteristic, and the like are improved. The compensation signal generating section 110 adds a self-aligning torque (SAT) 113 to an inertia 112 at an adding section 114. The addition result is further added with a convergence 111 at an adding section 115. The addition result at the adding section 115 is processed as the compensation signal CM.
In a case that the motor 20 is a three-phase brushless motor, details of the PWM-control section 105 and the inverter 106 have a configuration as shown in FIG. 3, and the PWM-control section 105 comprises a duty calculating section 105A that calculates the PWM-duty values D1 to D6 which are used in a three-phase PWM-control by using the voltage control command value Vref in accordance with a predetermined equation, and a gate driving section 105B that drives gates of the FETs as the driving device by means of the PWM-duty values D1 to D6 and turns-ON or turns-OFF the gates of the FETs for compensating a dead time. The inverter 106 is constituted by the three-phase bridge of the FETs (FET 1 to FET 6) as the semiconductor switching device, and the motor 20 is driven by turning-ON or turning-OFF the gates of the FETs by means of the PWM-duty values D1 to D6. A motor relay 23 for supplying (ON) the electric power or blocking (OFF) the electric power is connected to respective phases in the electric power supply lines between the inverter 106 and the motor 20.
With reference to such an electric power steering apparatus, unpredictable state at a system-abnormality detection time (for example, disconnection in the torque sensor, a short circuit of the motor control stage-FETs and the like) can be occurred. As a response in the above case, the assist-control of the electric power steering apparatus is immediately stopped, and a connection between the driving control system and the motor is blocked with the highest priority.
Generally, as shown in FIG. 3, the motor relay is interposed between the motor 20 and the inverter 106 that controls a current which is passed through the motor 20. A non-expensive contact relay is used to the motor relay 23, and the current which is passed through the current is blocked by electromagnetically releasing the contact point by means of the hardware (for example, Japanese Unexamined Patent Publication No. 2005-199746 A (Patent Document 1)).
However, recently, in order to miniaturize the apparatus, improve a reliability and decrease costs, the contact electromagnetic motor relay replaces with the contactless motor release switch that comprises, for example, the FETs (analog switch). However, when it is impossible to continue the assist control by the system abnormality, in a case that the motor is rotating even when the inverter is stopped, the motor release switch is turned-OFF in rotating the motor, the regenerative electric power of the motor deviates from the area of safety operation, and then the motor release switch is damaged and is destroyed. This problem is also occurred when the voltage is recovered and the switch is turned-ON again after the MCU is reset and the motor release switch is turned-OFF.
For example, Japanese Unexamined Patent Publication No. 2013-183462 A (Patent Document 2) discloses the apparatus that uses the semiconductor switching device as the motor relay. In the apparatus of Patent Document 2, when a failure of the electric power converter (the inverter) is detected, the driving of the inverter is stopped, and a first power supply relay and a second power supply relay are turned-OFF. In a state that the driving of the inverter is stopped, when the motor rotates by the external force and the regenerative voltage is generated, the regenerative voltage is regenerated from the inverter to the power supply (the battery) through parasitic diodes of the first power supply relay and the second power supply relay that are an ON-state.
In the electric power steering apparatus, we especially pay attention that the device is destroyed by the deviation from the area of safety operation of the semiconductor device by generating the motor back-EMF by means of the rotation of the motor and the switching loss when the motor relay is turned-OFF by the motor regenerative current. It is strongly desired that a countermeasure of the destruction in an on-vehicle device is non-expensively and surely performed without adding the hardware components as much as possible.
Further, in Japanese Patent No. 5120041 (Patent Document 3), in a case that all of phase release means (the motor relay) is opened, and the voltage in the one particular phase is applied, when the terminal voltage based on the applying of the voltage in the phase that is not the above particular phase is detected, it is judged that the short circuit is occurred in the phase release means in the above particular phase. Thus, the apparatus of the Patent Document 3 is detected for the failure of the phase release means itself, and does not protect the semiconductor switching device.